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What is a Catadioptric Telescope?

A catadioptric telescope is an optical system that is optimized for producing images of objects at an infinite distance, and which incorporates both refractive type optics (lenses) and reflective optics (mirrors). The use of both mirror and lens optics produces certain advantages in performance as well as in the manufacturing process. The term "catadioptric" results from two separate words: "catoptric" referring to an optical system which uses curved mirrors, and "dioptric" referring to one which uses lenses.

The four catadioptric telescope designs most commonly used by amateur astronomers are:

1. Schmidt-Cassegrain

2. Maksutov-Cassegrain

3. Schmidt-Astrograph

4. Schmidt-Newtonian

Schmidt-Cassegrain Telescope Design

The Schmidt-Cassegrain telescope has been one of the most popular designs offered to amateurs for many years. A Schmidt-Cassegrain telescope (SCT) in its usual configuration consists of a short tube with a concave spherical primary mirror, a full-aperture corrector lens, and a smaller convex secondary mirror located on the optical axis near the center of the corrector plate. The image is formed behind the primary mirror, which has a central perforation. The secondary mirror reflects the light cone from the primary mirror back through that perforation to the outside of the scope, and through a focus mechanism to the eyepiece used to view the image. This location of the image is why it is called a “Cassegrain” scope and it is very convenient for the astronomer either for visual use or astrophotography.

In the Schmidt-Cassegrain category, optical modifications have been made by some manufacturers to greatly reduce the aberrations of off-axis coma and focal field curvature that normally occur in the conventional Schmidt-Cassegrain design. These proprietary modifications allow the resulting telescopes to produce sharp star images with virtually no coma out to the edge of the field of view with minimal vignetting. This flat image field is ideal for use with modern imaging cameras.

One such proprietary telescope is the Celestron EdgeHD.  In this design, the primary and secondary mirrors retain the smooth spherical surfaces found in the Schmidt-Cassegrain and the corrector plate remains unchanged as well. However, Celestron designed the EdgeHD with two additional internal lenses placed close to the primary mirror to do the job of correcting aberrations. The result is a flat field of sharp images without off-axis coma right out to the edge of the field.

Meade Instruments has their own proprietary design to greatly reduce off-axis coma. The  Meade ACF, or Advanced Coma-Free telescopes retains the spherical primary mirror of their original Schmidt-Cassegrain telescope, but the secondary was replaced with a hyperbolic-figured secondary mirror and the figure of the full-aperture corrector plate was changed to produce a combined optical system that results in a flat field of sharp images right out to the edge of the field.

Maksutov-Cassegrain Telescope Design

The Maksutov-Cassegrain telescope is also a very popular design offered to amateur astronomers. In its usual configuration, the Maksutov-Cassegrain consists of a short tube with a spherical concave primary mirror, a full-aperture corrector lens which is a weak negative meniscus lens, and a small convex secondary mirror on the inside of the corrector plate. It takes the name “Cassegrain” because its image is formed outside behind the primary mirror. Maksutov-Cassegrain telescopes (often called “Mak” for short) have been popular because they offer sharp,  high magnification images in a short tube that is easy to use and requires little in terms of user maintenance.

Schmidt Astrograph Telescope Design

The catadioptric Astrograph is a telescope designed for doing astrophotography rather than visual observing. In amateur astronomy Astrographs are used mostly for obtaining images of various objects, but they also have been used for doing sky surveys as well as searching for comets or asteroids. Regardless of their specific optical design, the Astrograph usually has similar characteristics, such as a low focal ratio (meaning shorter focal lengths than other telescopes) and a wide field of view that displays sharp images. Often accessory lenses, such as focal reducers and field flatteners, are used with these fast, wide field telescopes to flatten their image field and/or reduce their focal length even further. Astrographs are made in various optical/mechanical configurations including Schmidt-Cassegrain and Schmidt-Prime-Focus. There are also modified-Cassegrain reflector scopes, but these do not normally use a full-aperture corrector plate.

Schmidt-Newtonian Telescope Design

Schmidt-Newtonian telescopes are a cross between the common Newtonian reflector telescope and a Schmidt-corrected Cassegrain. They create the image on the side of the tube, closer to the front aperture just like the Newtonian. They have a concave spherical primary mirror and an aspheric corrector lens placed near the entrance aperture of the telescope tube. Their flat secondary mirror is often attached to the center of the corrector lens, eliminating the need for a mechanical spider that is normally found in classic Newtonian reflectors. This flat secondary mirror does not add any power to the system. These telescopes tend to have shorter focal lengths, resulting in wider fields of view compared with those obtained with conventional Newtonian reflectors.

More Information on the Catadioptric Design

Catadioptric telescope designs (which combine both lenses and mirrors) may provide better aberration correction than other all-lens or all-mirror telescopes over a wider aberration-free field of view, but their principle advantages for the amateur astronomer are in mechanical size and weight reduction. For manufacturers of these scopes, their ability to use mirrors with purely spherical figures and easily reproduced refractive elements, usually called corrector lenses, results in lower manufacturing costs, which in turn can lower the purchase price for the consumer.

Most of the catadioptric telescope designs in common use today have primary and secondary mirrors with spherical surface figures. Front surface spherical mirrors are easier to reproduce in quantity in manufacturing facilities than parabolic or hyperbolic mirrors, which also lowers their fabrication cost. The associated corrector lenses they require are usually single-element designs. Manufacturers have learned how to reproduce these in ways which reduce their cost as well.

All telescopes which utilize both mirror and lens optics require some type of corrector lens when the primary mirror is a spherical surface design. This is because a spherical mirror produces an image of an infinite object which is not perfect due to a form of optical distortion known as spherical aberration. Fortunately this aberration can be corrected by placing a single-element lens of special design at a particular distance in front of the mirror. This corrector plate, as they are commonly called, refracts (bends) the incoming rays of light from a star by a small amount before they reach the mirror. This limited amount of refraction is enough to remove the effects of spherical aberration to produce a sharp image of the star.

Because catadioptric telescope designs use a corrector lens (or plate) in fairly close proximity to the primary mirror, the result is an overall telescope tube length which is shorter than tubes required by other systems, such as all-mirror designs. Shorter tubes result in lighter weight for the telescope, which is another benefit for the user. In many cases, the light beam reflected by the primary mirror is intercepted by a secondary mirror placed at or near the corrector and reflected back down through a hole in the primary mirror. The image is then formed outside of the telescope tube behind the primary mirror. This is a useful mechanical result. It allows the eyepiece or other external equipment to be placed behind the scope, and supported by a fairly robust mechanical cell which holds the primary mirror.

This fact also makes it easier to balance the telescope on its mount. These factors: shorter tube length, lighter weight, better balance options, convenient location of the eyepiece, and lower cost combine to benefit the amateur astronomer and have made catadioptric scopes very popular with astronomers and other consumers.

More on the Schmidt-Cassegrain

The Schmidt-Cassegrain telescope has been one of the most popular designs offered to amateurs for years. In its classical configuration, a Schmidt-Cassegrain consists of a short tube with a spherical concave primary mirror, a full-aperture corrector lens, and a smaller convex secondary mirror located on the optical axis near the center of the corrector plate.  The image is formed behind the primary mirror, which has a central perforation. The corrector lens looks flat to a casual observer, appearing to be a window, but it has a subtle figure, called an asphere, that is produced by varying its thickness as a function of its radius. This aspheric lens was invented by Bernhard W. Schmidt in 1930. His original application of this design was for a "pure" Schmidt camera, which will be discussed later.

In the typical Schmidt-Cassegrain telescope for amateur astronomers, the secondary front-surface mirror is convex and nearly spherical. This secondary mirror may have its figure altered slightly during manufacturing to improve the image quality, thus "tuning" the optical performance of the telescope without altering the primary mirror or the corrector. The concave spherical primary mirror typically has a "fast" or short focal length, equivalent to a low f-number. But the convex secondary mirror is designed to fold the cone of light back toward the perforation in the primary with a larger f-number. This “trick” increases the actual focal length of the telescope beyond what it would be had there been only a primary mirror.

Most Schmidt-Cassegrain telescopes provide a high magnification at the aerial image plane (the image formed in space behind the rear mirror cell) with a somewhat narrow field of view. The user observes this (aerial) image with an eyepiece. The typical Schmidt-Cassegrain telescope focuses its image by allowing the user to move its primary mirror slightly forward or backward along the optical axis, using a simple mechanical mechanism and knob.

More on the Maksutov-Cassegrain

The Maksutov-Cassegrain is named after Dmitri D. Maksutov, a Russian optician, who patented his design in 1941. In its classical configuration, it consists of a short tube with a spherical concave primary mirror, a full-aperture corrector lens which is a weak negative meniscus lens, and in most cases, it has a small convex secondary mirror formed by evaporating an aluminum mirror spot on the optical axis on the inside of the corrector plate. It takes the name “Cassegrain” because its image is formed behind the primary mirror which has a central perforation. The corrector lens on a Maksutov-Cassegrain looks obviously concave to the casual observer. This telescope design is often referred to as a "Spot Maksutov" or a "Gregorian-Maksutov" because of the spot of aluminized mirror placed on the inside of its corrector lens.

A Maksutov-Cassegrain telescope typically produces a high magnification at the aerial image plane behind the mirror cell with a somewhat narrow field of view. The user observes this aerial image with an eyepiece. The Maksutov corrector lens is a meniscus, meaning it has two spherical curved surfaces, convex on one side facing toward the primary and concave on the other side. The spherical figure of both surfaces makes it easier to manufacture in quantity. Maksutovs employ a thick weakly diverging but strongly curved lens at a point in the optical path ahead of the primary mirror to produce an amount of spherical aberration that is equal but opposite in sign to that of the primary mirror. Chromatic aberration is corrected by making the meniscus lens slightly diverging.

Some Maksutov-Cassegrain telescopes focus their image by allowing the user to move the primary mirror slightly forward or backward along the optical axis, similar to the Schmidt-Cassegrain design. To eliminate the problem of image shifting during focusing and also to eliminate optical alignment tasks (often called collimation) for the user, the primary mirror can be perfectly aligned during manufacturing process and then fixed securely within its cell. The user then focuses the telescope image by moving the eyepiece via some mechanical method. This design was popular in some early Russian-made Maksutov-Cassegrain telescopes.

The Spot Maksutov-Cassegrain telescope also has another advantage over the typical Schmidt-Cassegrain made for amateur astronomy applications. The diameter of the secondary mirror spot tends to be smaller than secondaries commonly used in Schmidt-Cassegrain telescope designs. This smaller-diameter central obscuration of the aperture improves the performance of the optics, for reasons beyond the scope of this particular article. This results in high contrast, small images which may approximate those produced by refractors when the aperture of the Maksutov-Cassegrain is larger than the aperture of the compared refractor telescope. This is generally true, although not always, again depending on the specific manufacturing design.

More on Schmidt Astrographs

A Schmidt Prime-Focus Astrograph is similar to a Schmidt-Cassegrain except the image is not folded back to the rear of the tube. Instead, the image is  formed at the prime focus, which is up near the front aperture of the tube, and therefore, a secondary mirror is not required. However, due to curvature of the image field and other small aberrations, these telescopes usually have a multi-element lens just before the focus to flatten the image field and correct any residual image aberrations. The imaging camera is placed just outside the front of the corrector plate to capture the images.

Astrographs tend to have fast optics (low f-numbers) since the cone of light is not folded back, and they have a much wider sharp field of view than typical Schmidt-Cassegrains, making them ideal for imaging larger celestial objects as well as for rich-field imaging. In addition they have short mechanical tubes. Astrographs can have other designs as discussed in the first section of this article.

More on Schmidt-Newtonians

While not as popular as the other catadioptric telescopes described above, the Schmidt-Newtonian is still worth a mention. This optical design is a cross-over between a more common Newtonian reflector telescope and a Schmidt corrected catadioptric design. In particular, the typical Schmidt-Newtonian telescope has a concave spherical figure primary mirror and an aspheric corrector lens placed near the entrance aperture of the telescope tube. As in other Schmidt-type catadioptric designs, the Schmidt-Newtonian sports a corrector lens that refracts the incoming light rays just enough to correct for the inherent spherical aberration of the primary mirror. The secondary mirror is a Newtonian-style flat that is placed at 45º to the optical axis, and serves to fold the converging beam from the primary mirror out through the side of the tube to the focal plane.

In a Schmidt-Newtonian telescope, the flat secondary mirror is often attached to the center of the corrector lens, eliminating the need for a mechanical spider that is normally found in classic Newtonian reflector telescopes. This feature also eliminates the optical diffraction spikes caused by the spider veins when observing bright objects. Additional features of this design include the fact that, assuming the telescope is well made and properly aligned, aberrations of coma, astigmatism and field curvature are lower than aberrations in a comparable Newtonian with a conventional paraboloidal primary.

Catadioptric Designs More Suitable for Professional Applications

This section briefly discusses those catadioptric optical designs which are useful in professional applications, such as large observatories, satellite tracking facilities, military applications and industry.

The original pure-Schmidt-corrected catadioptric telescope invented by Bernhard W. Schmidt used a concave spherical primary mirror and an aspheric corrector lens (plate) located at the center of curvature of the primary mirror.  In order to achieve, low f-numbers (short focal lengths), very wide fields of view, and control coma and astigmatism aberrations, the focus was located inside the mechanical tube of the instrument. Placing the focus within the actual tube is a workable compromise for very large telescopes or "cameras" used by professionals. However this would be a serious problem for amateurs and their smaller instruments.

Further complicating the use of large pure-Schmidt scopes and cameras is the fact that the focal surface within the mechanical tube is typically not flat, hence it is not a "focal plane". This annoying feature was acceptable, however,  in order to gain the very wide field of view which it provided. For example, one of the more famous large Schmidt cameras used in professional astronomy is the 48" Samuel Oschin Schmidt Telescope at Palomar Observatory which was used in the National Geographic Society - Palomar Observatory Sky Survey. This instrument had a curved focal surface and was used to photograph large areas of the celestial sphere with photographic glass plates.

These photographic devices were large glass plates with photosensitive emulsions formed on one side. The plates were made by Eastman Kodak, typically to astronomers specifications, and were sensitive to different parts of the spectrum based on the task at hand. They were purposely made very flat. When used with the 48" Oschin Schmidt Telescope (originally called the Palomar Schmidt Telescope) the glass photographic plates were installed in a special holder which bent the glass plates slightly (while supporting them) to force the emulsion to correspond to the curved focal surface of the Schmidt camera. Remarkably this worked most of the time, although shattered glass plates did occasionally occur.

Another catadioptric system which was used in professional observatories was the Baker-Nunn design, by Dr. James Baker and Joseph Nunn. This design replaced the Schmidt corrector plate with a triplet corrector lens closer to the focus of the camera. The most popular Baker-Nunn camera design was used by the Smithsonian Astrophysical Observatory to track artificial satellites. This camera had a 20" aperture, worked at f/0.75 (a remarkably fast system) and used 55 mm wide film as a sensor (from the "Cinemascope 55" motion picture process). That Smithsonian Astrophysical Observatory Baker-Nunn camera example weighed about 3.5 tons, and was supported on a sophisticated yoke mount allowing it to track satellites which could occur in various orbits.

Miscellaneous Optical Applications:  Catadioptric systems have been used in searchlights and lighthouse beam projection. The often used "Fresnel lens" lighthouse illumination system incorporates a Fresnel lens made of glass segments as part of its focusing optics to help concentrate its light output into a narrow beam.

A few manufacturers have produced telephoto lenses for film cameras which are essentially small-scale catadioptric telescopes made to attach directly to specific lens mounts on single-lens reflex (SLR) cameras. These lenses, often called mirror-lenses, are suitable for telephoto applications because they incorporate fairly high magnifications, but typically they do not have adjustable iris apertures. Most do have a variable-focus adjustment.

Catadioptric systems have been applied in headlamps, military laser beam projectors, and have been used in high-end microscopes. Some infrared (IR) surveillance optics use specialized catadioptric systems incorporating IR transmitting lens materials.